The thumb-brush technique is an accompaniment pattern that has a "boom-chick" sound. Here, the thumb plays normally (plucking a bass string downward), but the fingers strike (brush) the top three or four strings with the backs of the nails in a downward motion (toward the floor). The fingers actually strum the strings as a pick does, but you don't move your arm or your whole hand.
Basically, you curl your fingers into your palm and then quickly extend them, changing from a closed-hand position to an open-hand position, striking the strings with the nails in the process.
This figure shows two measures of the thumb-brush pattern on a C chord. Don't worry about hitting exactly three strings with the finger brush. Getting a smooth, flowing motion in the right hand is more important. Watch Video Clip 67 to see how the right-hand fingers brush the strings.
A variation of the simple thumb-brush is the thumb-brush-up (which yields a "boom-chick-y" sound). After strumming with the backs of the nails of the middle and ring fingers, you use the flesh of your index finger to pluck the 1st string (upward). You invariably perform this technique in an eighth-note rhythm on beats 2 and 4 (one, two-and, three, four-and).
The following figure shows a two-measure pattern, using the thumb-brush-up technique. Keep the downstrokes and upstrokes steady, with no break in the rhythm. (Make sure you listen to Track 84 for this one, and don't be discouraged if this pattern takes a little getting used to.)
Don't think of the upstroke with the finger as a fingerpicking move but as an upward brush with the whole hand. In other words, keep the right hand loose and flowing as you pull it upward to strike the 1st string with your 1st finger.
You can use the thumb-brush or thumb-brush-up pattern for any song that has a "boom-chick" or "boom-chick-y" sound, such as "Jingle Bells" or "I've Been Working on the Railroad."